Studies of the micronutrients zinc, manganese and silicon in cucumbers (Cucumis sativus)

Abstract:

Zinc and manganese have long been considered as essential micronutrients to plant
growth, yet the interactions of the two nutrients on growth and development of plants have not
been elucidated in their entirety. Silicon is not classed as an essential element, but has been
found to improve growth of a number of crops, particularly of the Poaceae family.
A simple water culture hydroponic system was developed to monitor the growth and
development of a fruit crop (Cucumber – Cucumis sativus) under deficient, adequate and
excessive applications of zinc and manganese. Plant growth parameters were monitored including
leaf growth, plant height, plant fresh and dry mass, yield, fruit size and fruit mass. Nutrient uptake
was also measured using inductively coupled plasma emission spectroscopy, whilst chlorophyll
was determined spectrophotometrically. Plant nutrient analyses were also conducted using
inductively coupled plasma emission spectroscopy.
Silicon was found to have a beneficial effect on the growth of cucumbers and was
incorporated as a treatment for this crop along with zinc and manganese since foliar silicon sprays
were able to correct the occurrence of mineral deficiency symptoms. Along with plant growth
measurements, nutrient uptake, plant nutrient analysis and chlorophyll determination, plant
tissue was also analysed using transmission electron microscopy to establish the impact of silicon
applications on the cell ultra-structure of cucumbers. Electron micrographs showed an increased
presence of plasmodesmata in treatments excluding silicon. Such increased plasmodesmata
connections under silicon deficient conditions could increase translocation of cell solutes due to
reduced cell longevity.
Results also confirmed the essentiality of zinc and manganese on plant growth and
development as typical deficiency symptoms were observed. Typical toxicity symptoms were also
recorded. Rates of uptake of nutrients corresponded with leaf growth and enlargement as well as
yield. The chlorophyll concentration was not a clear indicator of nutrient application level.
Typically, manganese and zinc interacted with iron, magnesium, calcium and potassium, affecting
their uptake into the plant dependent on the level of manganese and zinc applied.
Although non-essential, silicon improved plant growth, but had neither a relationship with
the other nutrients evaluated nor affected the physical growth and development of the plants.
Manganese and zinc, as essential to plant growth and development, affect the visual appearance
of the plant as well as affect the plant biochemically due to their involvement in many growth and
development processes.